Glycolic acid (HOCH2COOH; CAS Registry Number is 79-14-1) is the simplest member of the a-hydroxy acid family of carboxylic acids. Its properties make it ideal for a broad spectrum of consumer and industrial applications, including use in water well rehabilitation, the leather industry, the oil and gas industry, the laundry and textile industry, and as a component in personal care products. Glycolic acid also is a principal ingredient for cleaners in a variety of industries. Such cleaners include dairy and food processing equipment cleaners, household and institutional cleaners, metals processing cleaners (for metal pickling, copper brightening, etching, electroplating, electropolishing), and industrial cleaners (for transportation equipment, masonry, printed circuit boards, stainless steel boiler and process equipment, and cooling tower/heat exchangers).
Glycolic acid can also be used to produce a variety of polymeric materials, including thermoplastic resins comprising polyglycolic acid. Resins comprising polyglycolic acid have excellent gas barrier properties, and such thermoplastic resins comprising polyglycolic acid may be used to make packaging materials having excellent gas barrier properties (e.g., beverage containers, etc.). However, it is known that impurities within glycolic acid, such as diglycolic acid, may adversely impact the gas barrier properties of resins comprising polyglycolic acid. As such, there is a need to produce high purity glycolic acid.
A variety of industrial processes are used to produce glycolic acid, and each typically introduces impurities to the final glycolic acid product. Such processes include carbonylation of formaldehyde, oxidation of ethylene glycol, chloroacetic acid saponification, acid hydrolysis of glycolonitrile (with an optional glycolic acid ester intermediate), hydrolysis of methyl glycolate, and enzymatic hydrolysis of glycolonitrile. For example, glycolic acid obtained by carbonylation of formaldehyde in water, in the presence of an acid catalyst contains glycolic acid dimer or oligomers formed by ester-forming dehydrocondensation of glycolic acid, and diglycolic acid as major impurities in addition to residues of the catalysts. Patent application WO92/05138 (European Patent 0552255B1) describes that a 70% technical grade glycolic acid aqueous solution typically shows the following composition by weight percent: glycolic acid 62.4%, glycolic acid dimmer 8.8%, diglycolic acid 2.2%, methoxyacetic acid 2.2%, and formic acid 0.24%.
Purification of a hydroxycarboxylic acid can be difficult as heating often results in the formation of polycondensation products. As such, a purification process that involves significant heating, such as distillation, is difficult.
The use of crystallization to obtain a more purified form of glycolic acid has been reported in the art. Patent application WO2006/064611AI teaches a single stage, high yield crystallization process (yield of 95.5%). However, the process provides only a moderate purity improvement for diglycolic acid (88%), and does not describe a multi-stage process characterized by both high yield and high purity. Patent application WO2003/64366A1 (corresponding to US Patent application 2005/20853A1) teaches a single stage crystallization process capable of providing a highly purified glycolic acid product, but with relatively low recovery yield.
U.S. Pat. No. 7,002,039 claims a method for purification of an a-hydroxy acid by subjecting the starting material to at least two batch crystallization steps with “both crystallization steps preferably being carried out in one device.” [column 4, lines 28-30]. This patent does not teach a continuous, multi-stage crystallization process having solid or liquid recycling, whereby high purity glycolic acid can be obtained in high yield.
Therefore a need still exists for a process that provides both high yield and high purity glycolic acid. The present invention provides such a process.